Most commonly, the current practice is to separate minerals of different density by gravity separation. Typical equipment used in the gravity separation operation includes jigs, shaking tables, sluices, spirals, and Reichert cones. However, gravity separation has a marked disadvantage. When the size of the particles to be separated out falls below 50 microns, gravity separation equipment either loses collection efficiency or has very low throughput rates. The former result renders the equipment useless; the latter result renders it more costly or expensive to operate.
The Reichert cone concentrator is a high-capacity gravity separator incorporating multiple stages of flowing film concentration that has found application in areas of mineral processing where the materials have different specific gravities. This concentrator operates by feeding slurry onto a first curved conical surface in an annular distribution pattern. The dispersed slurry flows naturally to the outside edge of the cone surface, then changes direction and moves inward along a concentrating cone surface. As the slurry gravitates toward the center of the concentrating surface, the bed thickens due to the progressively smaller area available. The finer, heavier particles gravitate to the cone surface by a combination of interstitial trickling and normal settling mechanisms under the influence of gravity. Under controlled flow conditions, a large proportion of the heavier particles tend to remain in the lower layers of the moving bed of slurry, close to the cone surface. The stratified layers are then separated by an annular slot. Dilution water is provided via an annular water ring.
The drawback of the Reichert cone concentrator is the numerous stages required for a high-concentration assembly. Also, a typical cone system only enables efficient recovery of particles greater than 40-50 microns in size.
As a result of the deficiencies of such gravity separators, centrifugal separators have been adapted for use in the processing of ore. The Yunnan Tin Mining Company in China reports the development of a batch-type centrifugal separator for separation of cassiterite particles. Recoveries reported were 75-90% for plus 10 micron particle and 35-40% for minus 10 micron particle. The throughput of one unit is reported to be 30-35 metric tons per day. This centrifugal separator has also been used in recovery of tungsten minerals. No detailed description of the equipment is known to applicant although it is reported to have shortcomings (e.g., excessive consumption of water and noncontinuous feed).
Several centrifugal jig devices have been developed which enhance concentration by means of gravity separation. These centrifugal jigs enable the separation of materials with relatively small specific gravity differences. Also, by negating the surface effects which mask differences in the specific gravities of tiny particles, the centrifugal jigs allow gravity concentration to be applied to smaller particle sizes.
A continuous-flow centrifugal jig or concentrator marketed by the Indeco Company comprises a rotating cylindrical jig bed and a system for pulsed injection of liquid. This unit is described in U.S. Pat. No. 4,279,741. In comparison to a conventional mineral jig which is essentially a combination of two types of gravity separation systems (the rising current classifier operating by pulsation of liquid in opposition to settling forces and the heavy media separator obtaining separation of particles with different specific gravities), the system disclosed in U.S. Pat. No. 4,279,741 enhances the operation of a mineral jig by providing for centrifugally forced settling of particles by rotation of an even, layered jig bed. The combination of centrifugally forced settling and positive pulsation of liquid is utilized in the area of the jig bed to separate pulp into a heavy and a light fraction. The main drawback of this concentrator is that it is a sophisticated and complicated unit that requires time-consuming setup and stringent control procedures. Also, because of the nature of the process, feed must be very carefully classified ahead of the concentrator. Therefore, the operation of the concentrator is further complicated by the use of an interdependent classifier specifically adapted to the strict requirements of the concentrator.
The hydrostatic separator made by Knelson also operates according to the principles of centrifugation, but is not a jig. Essentially this unit comprises a high-speed ribbed rotating conic bowl with a drive unit. Feed material is fed into the spinning bowl. Under the influence of centrifugal force, concentrates collect in the ring-divided zones on the periphery of the bowl while the lighter tailings are spun upward along the slope of the bowl and overflow the rim. The unique aspect of this centrifugal concentrator is that a flow of water is injected through graduated perforations in the bowl wall. The injected water fluidizes the trapped concentrate, preventing compaction, which allows the bowl to be rotated at a much fastener rate. Thus, higher centrifugal forces are produced which enable even very fine particles of gold to be separated. The concentrate is collected in the bowl and emptied once a day. Thus, this separator is specifically designed to recover rare heavy particles such as gold and has little use as a continuous separator of light and heavy fractions because of the limited storage capacity for the heavy fraction and the excessive delay which would attend frequent stoppages for draining the concentrate.
The Tobie centrifugal concentrator is used by Koapsche Diggings in Transvaal for recovery of gold from gravel by gravity separation. Feed water are supplied to a drum rotating at 84 rpm. Float discharge in the rotating drum advances down the sloping drum and is discharged by means of internal lifters. The principle of operation is that gold particles are dense enough to be held against the wall of the drum by centrifugal force while the less dense material in the water passes through the system. At the end of the workday, the gold is removed from the drum. As was the case with the Knelson hydrostatic separator, due to the limited capacity of the drum, continuous flow separation of light and heavy fractions would not be practical with the Tobie centrifugal concentrator if the heavy fraction being concentrated was not highly dense and highly valuable.
Continuous-flow imperforate basket centrifuges can be used for classification by size. In this type of centrifuge, a helical conveyor moves the centrifuged solids along the inside surface toward the smaller diameter of a spinning frustum of a cone. However, the conveyor moving through the solids tends to mix them and prevent separation into laminae of particles according to density. This drawback makes such a centrifuge inefficient for the separation of light and heavy fractions.
Finally, the ultracentrifuge is a laboratory tool typically used for separating colloids and polymers of varying size and density. The unit operates with high centrifugal force, low percentage of solids, and in a particle size range smaller than that separated by the centrifugal separator of the present invention. Because the throughput rate of an ultracentrifuge is small, it has no applicability for commercial recovery of minerals.